Asda, the supermarket chain, may have to p-p-pick up a bill for damages after the judge in the great chocolate biscuit case ruled that its Puffin brand was a copy of the famous Penguin.

Mr Justice Robert Walker found Asda guilty of passing-off, but allowed the supermarket to continue making its biscuits provided it amended the packaging. United Biscuits had sued Asda on the grounds that the supermarket's product was a copy of its 60-year-old brand.

The seven-day hearing at the High Court in London was supposed to end the confusion of the shopper but ended up baffling the biscuit-makers, both of which claimed a victory. Asda said it would continue to sell Puffins after "tweaking" the packaging, although it admitted that it was unsure what changes the judge had in mind.

Mr Justice Walker's 35-page judgment included his "judicial notice of relevant ornithology". After noting the differences between the two birds' Latin names, plumage and nesting places, he admitted: "My own knowledge has been supplemented considerably from a handbook which counsel showed me."

In a judgment punctuated by pictures of Puffins and Penguins, the judge referred to various images used by UB over the years, including Penguins in rubber boots, chef's hat and ice skates and scarf. Recently UB had gone back to using more naturalistic penguins - "not encumbered by boots, skates or other paraphernalia".

The judge said that had the Asda product been called "Bison" with an appropriate cartoon, the case would never have been brought. "The word Puffin is not very different in form from Penguin," he observed, and concluded: "The Puffin packaging and get up was, in the material sense, deceptively similar to those of the Penguin." He ordered an inquiry into the amount of damages suffered by UB as a result of the Asda product.

UB's sales of McVities Penguins amount to pounds 30m a year with more than pounds 4m spent annually on advertising. Asda's expenditure on developing and launching its rival last September was also substantial. The Puffin - 25 per cent cheaper than the Penguin - was introduced at its 200-plus stores as a "brand beater".

The judge cleared Asda of infringing UB's registered trade marks - the name and pictures of the bird - except for two incidents when the supermarket went so far as to use the phrase "Pick up a Puffin" in its advertising. He granted a 35-day stay of execution to hundreds of thousands of Puffins awaiting sale in the old packaging.